Paytm not working on my iPhone 8 with iOS 16

The iPhone 8 will only run up to iOS 16.7.11, released in March 2025. If what you wish to do will only work with a newer iOS, then you will need to get a newer iPhone that will support a newer iOS. You will have to see what iOS is required for paytm. If it supports iOS 16.7.11, then you need to contact their support to see what the problem is. That is not an Apple product.

It would appear that while PayTm's main function is handling UPI payments, it might have some non-UPI functions. That's probably the only reason that the App Store description says that it requires "iOS 14.0 or later".

• You can install the app on any iPhone running iOS 14.0 or later.
• You can use many or all of the non-UPI functions on any iPhone running iOS 14.0 or later.
• But you can only use the UPI functions on an iPhone running iOS 17.0 or later. Your iPhone 8 cannot update past iOS 16.*, which means, that in effect, the people running the UPI system have abandoned support for your phone.

I believe about all UPI Payment apps require iOS 17 or above. The reason for this is likely because UPI payment fraud is very prevalent in India, so the NPCI asked smartphone companies to combat this fraud. Apple released an update, and it came in iOS 17. And the NPCI was so willing to discontinue support for older iPhone models.